1. Field of the Invention
The invention relates in general to a method of testing a transistor, and more particularly to a method of testing a kink effect of a transistor.
2. Description of the Related Art
During the fabrication of an integrated circuit (IC), a kink effect usually occurs during the formation of shallow trench isolation. Hydrogen fluoride is usually utilized to etch away the undesired silicon oxide but it also damages the field region. The damaged field region therefore is unable to isolate the adjacent conductive layers completely. Consequently, a sub-threshold current arises, which strongly effects the quality of the ICs. This is what is known as the kink effect. Kink effect testing is therefore necessary during IC fabrication.
Referring to FIG. 1A, a schematic top view of a transistor, the source/drain regions 100a and 100b are located at two sides of the polysilicon gate 102a, and the contact window 104 is formed for connecting the 20 source/drain region and other transistors. When no kink effect exists, the width of the polysilicon gate 102a remains uniform, the current is steady and sub-threshold current is eliminated.
However, if the polysilicon gate 102b is damaged, as shown in FIG. 1B, and has a varied width, the threshold current becomes unstable. It is proposed that a pseudo-transistor B be formed, and have a different threshold voltage from the original transistor A. Therefore, the undesired kink effect occurs due to the existence of the pseudo-transistor B.
Referring to FIG. 1B, the width of the polysilicon gate of the transistor A is assumed to be L and that of the pseudo-transistor B is assumed to be L'. If L is equal to L', the threshold is steady, which indicates that the quality of the transistor is high. If L is larger than L', the pseudo-transistor B turns on before the original transistor A, which results in a lower sub-threshold current. In contrast, if L is smaller than L', the dielectric effect of the transistor A is reduced, the transistor A turns on earlier than the pseudo-transistor B and, consequently, leakage current appears. To sum up, it is apparent that if the whole transistor has an unequal polysilicon gate width, it is poor in quality.
The conventional method for testing for the existence of a kink effect includes providing the gate with a gate voltage Vg to obtain a corresponding source/drain current Ids, and drawing a curve as a function of the source/drain current versus gate voltage (Ids-Vg). Referring to FIG. 2B, curve 206 is obtained from a transistor with a kink effect and curve 208 is obtained from a transistor without a kink effect. It is shown that the curve 208 of the ideal transistor has only one turning point, while the curve 206 includes more than one turning point. This result is then used to determine the existence of kink effect. However, this conventional testing method can be used for qualitative analysis but not for quantitative analysis, and the degree of the kink effect cannot be found by this method. Moreover, this method cannot be used at the 0.25 .mu.m level.